Make "FutureIterator" queue management more formal

Summary: Depends on D21035. Ref T11968. This allows a "FutureIterator" to hold futures which are blocked because of unresolved dependencies, and makes the resolution process more structured.

Test Plan: Ran unit tests, created this revision.

Maniphest Tasks: T11968

Differential Revision: https://secure.phabricator.com/D21036